This invention relates in general to an improved hydraulic brake system for use in a vehicle, and in particular to an anti-lock braking system.
Vehicle braking systems generally include a driver operated brake pedal which is connected to a master cylinder. The master cylinder is in fluid communication with brakes at associated wheels of the vehicle. Operation of the brake pedal generates pressure within the master cylinder which is supplied to the brakes, thereby decelerating the vehicle or preventing the vehicle from moving from a stopped position. A booster may also be provided between the brake pedal and the master cylinder to assist the pedal effort by the driver of the vehicle.
Typical anti-lock braking systems (ABS systems) further include a separate hydraulic control unit (HCU) in fluid communication between the master cylinder and selected brakes. The HCU incorporates a plurality of solenoid operated valves to selectively control the application of hydraulic fluid pressure to the selected brakes to prevent locking of the associated wheels. Generally, the valves cyclically relieve and reapply pressure to the associated brakes to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver.
ABS systems can be configured to independently control the application of fluid to all four brakes associated with a four wheeled vehicle. Alternatively, an ABS system can be configured to control the application of fluid to the brakes associated with the rear wheels of the vehicle, such as the ABS system disclosed in U.S. Pat. No. 4,828,335 to Fuller et al.
Typically, the HCU of an ABS system includes a fluid accumulator for the storage of fluid which is diverted from the wheel brakes during an ABS function. A conventional accumulator includes a variable volume fluid reservoir which is maintained at a slightly elevated pressure by a slidable piston disposed therein. The accumulator includes a spring which biases the piston. Fluid enters the reservoir of the accumulator causing the piston to move against the biasing of the spring, thereby increasing the size of the reservoir to store fluid. If one or more of the solenoid valves for selectively controlling the application of pressure to the brakes is faulty such that fluid leaks therethrough, the accumulator will generally fill up, thereby preventing further loss of fluid from the controlled brake circuit. Due to the size of the accumulator and the solenoid valves, the HCU is generally located remotely from the master cylinder and is in fluid communication therewith by a series of brake fluid conduits and connections. However, the accumulator adds weight and cost to the brake system.